nucleotides and nucleic acids Flashcards
nucleotides
DNA and RNA are nucleic acids: polymers that are made up of many repeating units (monomers) called nucleotides
Each nucleotide is formed from:
A pentose sugar (a sugar with 5 carbon atoms)
A nitrogen-containing organic base
A phosphate group
dna nucleotides
The components of a DNA nucleotide are:
A deoxyribose sugar with hydrogen at the 2’ position
A phosphate group
One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or thymine(T)
rna nucleotides
The components of an RNA nucleotide are:
A ribose sugar with a hydroxyl (OH) group at the 2’ position
A phosphate group
One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or uracil (U)
The presence of the 2’ hydroxyl group makes RNA more susceptible to hydrolysis
This is why DNA is the storage molecule and RNA is the transport molecule with a shorter molecular lifespan
purines and pyrimidines
The nitrogenous base molecules that are found in the nucleotides of DNA (A, T, C, G) and RNA (A, U, C, G) occur in two structural forms: purines and pyrimidines
The bases adenine and guanine are purines – they have a double ring structure
The bases cytosine, thymine and uracil are pyrimidines – they have a single ring structure
phosphodiester bond
DNA and RNA are polymers (polynucleotides), meaning that they are made up of many nucleotides joined together in long chains
Separate nucleotides are joined together via condensation reactions
These condensation reactions occur between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide
A condensation reaction between two nucleotides forms a phosphodiester bond
It is called a phosphodiester bond because it consists of a phosphate group and two ester bonds
The chain of alternating phosphate groups and pentose sugars produced as a result of many phosphodiester bonds is known as the sugar-phosphate backbone (of the DNA or RNA molecule)
As the synthesis of polynucleotides requires the formation of phosphodiester bonds, the same is true for the reverse process: the breakdown of polynucleotides requires the breakage of phosphodiester bonds
structure of atp and add
All organisms require a constant supply of energy to maintain their cells and stay alive
In all organisms this energy is required for:
Anabolic reactions (building larger molecules from smaller molecules)
Moving substances across the cell membrane or moving substances within the cell
In animals energy is also required for:
Muscle contraction – to coordinate movement at the whole-organism level
The conduction of nerve impulses
In all known forms of life, ATP from respiration is used to transfer energy in all energy-requiring processes in cells
This is why ATP is known as the universal energy currency
Adenosine Triphosphate (ATP) is a nucleotide
The monomers of DNA and RNA are also nucleotide
atp
Adenosine triphosphate (ATP) is the energy-carrying molecule that provides the energy to drive many processes inside living cells
ATP is another type of nucleic acid and hence it is structurally very similar to the nucleotides that make up DNA and RNA
It is a phosphorylated nucleotide
Adenosine (a nucleoside) can be combined with one, two or three phosphate groups
One phosphate group = adenosine monophosphate (AMP)
Two phosphate groups = adenosine diphosphate (ADP)
Three phosphate groups = adenosine triphosphate (ATP)
dna structure
The nucleic acid DNA is a polynucleotide – it is made up of many nucleotides bonded together in a long chain
DNA molecules are made up of two polynucleotide strands lying side by side, running in opposite directions – the strands are said to be antiparallel
Each DNA polynucleotide strand is made up of alternating deoxyribose sugars and phosphate groups bonded together to form the sugar-phosphate backbone. These bonds are covalent bonds known as phosphodiester bonds
The phosphodiester bonds link the 5-carbon of one deoxyribose sugar molecule to the phosphate group from the same nucleotide, which is itself linked by another phosphodiester bond to the 3-carbon of the deoxyribose sugar molecule of the next nucleotide in the strand
Each DNA polynucleotide strand is said to have a 3’ end and a 5’ end (these numbers relate to which carbon on the pentose sugar could be bonded with another nucleotide)
As the strands run in opposite directions (they are antiparallel), one is known as the 5’ to 3’ strand and the other is known as the 3’ to 5’ strand
The nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stranded DNA molecule
hydrogen bonding
The two antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by hydrogen bonds between the nitrogenous bases
These hydrogen bonds always occur between the same pairs of bases:
The purine adenine (A) always pairs with the pyrimidine thymine (T) – two hydrogen bonds are formed between these bases
The purine guanine (G) always pairs with the pyrimidine cytosine (C) – three hydrogen bonds are formed between these bases
This is process is known as complementary base pairing and the pairs are known as complementary base pairs
double helix
DNA is not two-dimensional as seen in the diagram above
DNA is described as a double helix (this refers to the three-dimensional shape formed by the twisting of the DNA molecule)
semi-conservative replication of DNA
Before a (parent) cell divides, it needs to copy the DNA contained within it
Doubling the DNA ensures that the two new (daughter) cells produced will both receive full copies of the parental DNA
The DNA is copied via a process known as semi-conservative replication (semi = half)
The process is called this because in each new DNA molecule produced, one of the polynucleotide DNA strands (half of the new DNA molecule) is from the original DNA molecule being copied
The other polynucleotide DNA strand (the other half of the new DNA molecule) has to be newly created by the cell
Therefore, the new DNA molecule has conserved half of the original DNA and then used this to create a new strand
the importance of retaining one original DNA strand
Retaining one original DNA strand ensures there is genetic continuity (i.e. genetic information is conserved) between generations of cells
In other words, it ensures that the new cells produced during cell division inherit all their genes from their parent cells
This is important because cells in our body are replaced regularly and therefore we need the new cells to be able to do the same role as the old ones
Replication of DNA and cell division also occurs during growth
semi conservative replication
DNA replication occurs in preparation for mitosis, when a parent cell divides to produce two genetically identical daughter cells – as each daughter cell contains the same number of chromosomes as the parent cell, the number of DNA molecules in the parent cell must be doubled before mitosis takes place
DNA replication occurs during the S phase of the cell cycle (which occurs during interphase, when a cell is not dividing)
The enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between the base pairs on the two antiparallel polynucleotide DNA strands to form two single polynucleotide DNA strands
Each of these single polynucleotide DNA strands acts as a template for the formation of a new strand made from free nucleotides that are attracted to the exposed DNA bases by base pairing
The new nucleotides are then joined together by the enzyme DNA polymerase which catalyses condensation reactions to form a new strand
The original strand and the new strand join together through hydrogen bonding between base pairs to form the new DNA molecule
This method of replicating DNA is known as semi-conservative replication because half of the original DNA molecule is kept (conserved) in each of the two new DNA molecules
dna polymerase
In the nucleus, there are free nucleotides which contain three phosphate groups
These nucleotides are known as nucleoside triphosphates or ‘activated nucleotides’
The extra phosphates activate the nucleotides, enabling them to take part in DNA replication
The bases of the free nucleoside triphosphates align with their complementary bases on each of the template DNA strands
The enzyme DNA polymerase synthesises new DNA strands from the two template strands
It does this by catalysing condensation reactions between the deoxyribose sugar and phosphate groups of adjacent nucleotides within the new strands, creating the sugar-phosphate backbone of the new DNA strands
DNA polymerase cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides)
Hydrogen bonds then form between the complementary base pairs of the template and new DNA strands
mutations
The replicated DNA molecules must be an exact copy of the parent DNA molecule, therefore the formation of the complementary strands must be a highly accurate process
Although the process is astonishingly accurate considering it is happening constantly in cells and at a considerable speed, occasional mistakes occur in the form of:
Bases being inserted into the complementary strand in the wrong order
An extra base being inserted by accident
A base being left out by accident
These mistakes in the process of semi-conservative replication of DNA result in the occurrence of random, spontaneous mutations (i.e. errors in the genetic code)
